I installed the auto executable GMXTWM5.00.30TW.exe package (Traditional Chinese, Simplified Chinese and English package) to the SD card.
When the sw.unl file was included in the SDCard\Garmin directory, it would come up with the error message you mentioned.
If I removed the sw.unl file, it worked via the WM6 menu interface but only in trial mode.
But if I went to SDCard\Garmin\Apps\WM and clicked on QuePPC.exe, it worked.
But that meant I could only enter Garmin this method and not via the WM6 menu interface.
So before running Garmin, I copied the following files from the SDCard\Garmin\Apps\WM to the Main\Garmin directory (the latter directory was created during the installation process):
QueBmp240.dll (as my phone resolution is 240)
All the files except QuePPC.exe replaced the ones in the Main\Garmin
I deleted Que.exe in the Main\Garmin directory then renamed QuePPC.exe as Que.exe.
It now works without the error message but first boots as Simplified Chinese (as I used the GMXTWM5.00.30TW.exe package). By going into the tools (the spanner icon), then settings (also a spanner icon), then display (PDA screen icon) and the third box down lets me then choose English.
Maybe I only needed to replace just the SDCard\Garmin\Apps\WM\QuePPC.exe into the Main\Garmin and rename it as Que.exe. But that is for others to test and report.
Maybe the above might also work with the GarminMobileXTforWindowsMobile_50020w.exe too. But first test whether you can bypass entering Garmin from the main WM6 menu interface ie. go into file explorer and double click on the SDCard\Garmin\Apps\WM\QuePPC.exe.